InGaP/GaAs HBT Technology is very attractive for use in many commercial applications for its excellent reliability and thermal stability. The first generation of InGaP-based power amplifiers for wireless handsets, wireless LAN, broadband gain blocks, and high-speed fiber optic products have been successfully developed and marketed. For future generations of these products, it is important to reduce the die size and cost as well as to provide additional functionality with improved circuit performance. The integration of bipolar (HBT) and field effect transistors (FET or HEMT) on the same chip offers a unique way to achieve these goals. While the combination of bipolar and field effect devices in an integrated circuit is well known in the silicon world (BiCMOS), there has been no viable way to realize this concept in GaAs-based technologies for large volume commercial applications.
Several methods of integrating AlGaAs/GaAs HBT with field effect devices have been discussed in the literature. In one approach described in Ho et al., “A GaAs BiFET LSI technology”, GaAs 1C Sym. Tech. Dig., 1994, p. 47, and D. Cheskis et al., “Co-integration of GaAlAs/GaAs HBT's and GaAs FET's with a simple manufacturable process”, IEDM Tech. Dig., 1992, p. 91, the HBT emitter cap layer is used as a FET channel. This approach had two major drawbacks. First, the emitter resistance of the HBT is high and second, the parasitic effect of the base layer degrades FET performance and limits its applications.
Another approach is to grow HBT and HEMT structures by selective MBE growth. (See Streit, et al., “Monolithik HEMT-HBT integration by selective MBE”, IEEE Trans. Electron Devices, vol. 42, 1995, p. 618 and Streit, et al., “35 GHz HEMT amplifiers fabricated using integration HEMT-HBT material grown by selective MBE”, IEEE Microwave Guided Wave Lett., vol. 4, 1994, p. 361.) The problem with this approach is the requirement of epi-growth interruption, wafer processing and epi re-growth. These steps render this approach un-manufaurable (i.e. high cost) with poor epi quality control.
It has also been shown that AlGaAs/GaAs HBT may be grown on top of the HEMT in a single growth run. (See K, Itakura. Y. Shimamolo, T. Ueda, S. Katsu, D. Ueda, “A GaAs Bi-FET technology for large scale integration”, IEDM Tech. Dig., 1989, p. 389.) In this approach the FET is merged into the collector of the HBT through a single epitaxial growth.
Several attempts have been also made to integrate InGaP/GaAs HBT with MESFET and HEMT. (See J. H. Tsai, “Characteristics of InGaP/GaAs co-integrated d-doped heterojunction bipolar transistor and doped-channel field effect transistor,” Solid State Electronics, vol. 46., 2002, p. 45 and Yang et al., “Integration of GalnP/GaAs heterojunction bipolar transistors and high electron mobility transistors”, IEEE Electron Device Lett., vol. 17, no, 7, July 1996, p. 363. In these approaches the channel of the field effect devices used an InGaP layer with low mobility and saturation velocity which results in high linear resistance and poor high frequency performance. These devices also show threshold voltages lower than −2 Volts. These characteristics, however, make them largely unsuitable for commercial applications.
There is therefore a need for methods and epitaxial structures for fabricating integrated pairs of GaAs-based HBT and FET devices that are suitable for commercial applications.